Applicants claim priority under 35 U.S.C. xc2xa7119 of German Application No. 199 18 808.4 filed Apr. 26, 1999. Applicants also claim priority under 35 U.S.C. xc2xa7120 of PCT/DE00/00919 filed Mar. 21, 2000. The international application under PCT article 21(2) was not published in English.
The application relates to a method for hardening the ring-shaped grooves of the head of a piston made of steel, of a steel piston, of a bottom part made of steel, or of steel piston heads by means of laser beams.
Such methods are known from U.S. Pat. No. 5,515,770 and JP-A 1-149424. It is important in connection with such methods, in the normal case, that the front edge of the groove is not melted down and that the bottom of the groove is not hardened because a higher level of stress exists there. In order to prevent the front edge of the groove from being melted down, the edge is kept shaded according to the JP-patent (FIG. 2) by means of a special device. The position of the laser beam is precisely specified in the U.S. patent.
The feature that is common to both methods is that the laser beam is incident upon the center axis of the piston in a radial manner, so that with an imagined extension of the laser beam, the latter intersects the center axis of the piston, and that a graphite coating increasing the absorption is applied before the hardening process is carried out.
The invention is dealing with the problem of preventing the bottom of the groove from being hardened in the application of methods of the type specified above. Said problem is solved by the characterizing feature of claim 1. Advantageous further developments of the invention are the objects of the dependent claims.
Any reflection of the laser beams in the direction toward the bottom of the groove is safely avoided by guiding the laser beam in an approximately tangential manner. According to claim 2, any reflection in the direction of the given opposite flank of the groove is safely avoided as well because the angle of impact is correspondingly smaller. Guiding the laser beam in a tangential manner means that the laser beams projected onto the plane of the ring-shaped grooves intersect the piston or head of the piston only peripherally in the area of its ring-shaped grooves as secants. In other words, the laser beam is directed onto the piston in about the peripheral direction.